| Description: | Spot welding is one of the oldest welding processes. It is used in a wide range of industries but notably for the assembly of sheet steel vehicle bodies. This is a type of resistance welding where the spot welds are made at regular intervals on overlapping sheets of metal. Spot welding is primarily used for joining parts that are normally up to 3 mm in thickness. Thickness of the parts to be welded should be equal or the ratio of thickness should be less than 3:1. The strength of the joint depends on the number and size of the welds. Spot-weld diameters range from 3 mm to 12.5 mm.
How spot welding works
Spot welding is one form of resistance welding, which is a method of welding two or more metal sheets together without using any filler material by applying pressure and heat to the area to be welded. The process is used for joining sheet materials and uses shaped copper alloy electrodes to apply pressure and convey the electrical current through the workpieces. In all forms of resistance welding, the parts are locally heated. The material between the electrodes yields and is squeezed together. It then melts, destroying the interface between the parts. The current is switched off and the "nugget" of molten materials solidifies forming the joint.
To create heat, copper electrodes pass an electric current through the workpieces. The heat generated depends on the electrical resistance and thermal conductivity of the metal, and the time that the current is applied. The heat generated is expressed by the equation: E=I2*R*t
where E is the heat energy, I is the current, R is the electrical resistance and t is the time that the current is applied.
Copper is used for electrodes because it has a low resistance and high thermal conductivity compared to most metals. This ensures that the heat is generated in the workpieces instead of the electrodes.
Materials suitable for spot welding
Steel has a higher electrical resistivity and lower thermal conductivity than the copper electrodes, making welding relatively easy. Low carbon steel is most suitable for spot welding. Higher carbon content or alloy steel tend to form hard welds that are brittle and could crack. Aluminium has an electrical resistivity and thermal conductivity that is closer to that of copper. However, aluminium's melting point is much lower than that of copper, making welding possible. Higher levels of current must be used for welding aluminium because of its low resistivity.
Galvanized steel (i.e. steel coated with zinc to prevent corrosion) requires a different welding approach than uncoated steel. The zinc coating must first be melted off before the steel is joined. Zinc has a low melting point, so a pulse of current before welding will accomplish this. During the weld, the zinc can combine with the steel and lower its resistivity. Therefore, higher levels of current are required to weld galvanized steel. |
